Structure of active species in alkali-ion-modified silica-supported vanadium oxide

Silica-supported vanadium oxide (V2O5/SiO2) and alkali-ion (Na+, K+, or Rb+)-modified V2O5/SiO2 samples have been characterized by diffuse reflectance UV-vis spectroscopy, Raman spectroscopy, X-ray absorption spectroscopy (XANES and EXAFS), and XPS. The structure of vanadium species in V2O5/SiO2 has been confirmed to be of a (V=O)O-3 tetrahedron. The addition of alkali ions to V2O5/SiO2 has caused neither the change of the coordination number of vanadium ions nor the formation of alkali vanadates on the silica support. In alkali-ion-modified V2O5/SiO2 samples, the structure of vanadium species has also been a VO4 tetrahedron, and the bond lengths of V-O have been longer than those of V2O5/SiO2. A rubidium ion has located at ca. 3.3 Angstrom from a vanadium atom and interacted with two V-O bonds directly. Ab initio molecular orbital calculations have indicated that the proposed structures of vanadium species in V2O5/SiO2 and alkali-ion-modified V2O5/SiO2 samples are reasonable. In addition, it has been clarified that by adsorption of water molecules, the structure of vanadium species in alkali-ion-modified V2O5/SiO2 samples is kept tetrahedral, while that in V2O5/SiO2 changes to a distorted octahedral one.